DESCRIPTION: HIV-1/AIDS remains a serious public health problem globally with an estimated 7,000 new infections each day. An effective HIV-1 vaccine will be necessary for the long-term control of the HIV-1 pandemic. The goal of this study is to develop a safe and effective vaccine to prevent HIV-1 acquisition and transmission. Several potent and broadly neutralizing monoclonal antibodies (bNAbs) derived from HIV-1 infected patients bind to high mannose carbohydrate structures on the gp120 subunit of the HIV-1 envelope (Env) protein, such as 2G12 and a series of newly identified antibodies PGT125-131 and PGT135 that are among the most potent and broadly cross-reactive bNAbs identified to date. The identification of these bNAbs suggests that the glycan shield of gp120 is a viable vaccine target. We have targeted the HIV-1 Env glycan shield by genetically modifying a yeast strain and have shown that the elimination of three carbohydrate processing enzymes in S. cerevisiae (TM for triple mutant) results in the uniform production of the Man8GlcNAc2 carbohydrate structures that are the major form of glycans in the epitopes recognized by the PGT bNAbs and 2G12. We have shown that immunization of rabbits with whole TM yeast cells as well as purified, highly glycosylated, 2G12/PGT128 reactive TM yeast proteins, without the use of any gp120 or HIV-1 related proteins, elicits antibodies that recognize synthetic high-mannose glycans as well as monomeric gp120 proteins from a wide array of HIV-1 isolates and all tested virions in a carbohydrate dependent manner. The genetic scaffold elicited antibodies also potently neutralize HIV-1 from different subtypes that carry high-mannose N-glycans although potent neutralizing activity to primary virus strains has not been observed so far. We hypothesize that appropriate presentation of high mannose clusters on heterologous proteins in a combination with optimized immunization strategies can elicit antibodies that specifically target the key neutralizing epitope on the glycan shield of HIV-1 Env and potently neutralize primary isolates, and so propose the following Specific Aims: 1. Select lead immunogens that can efficiently support PGT bNAb binding from genome-wide screens of highly glycosylated proteins using a bioinformatics approach in combination with the genetically modified TM yeast for immunization trials. 2. Optimize immunization strategies and elicit neutralizing antibodies using a stepwise scaffold prime-Env boost strategy with the lead immunogens and a soluble HIV-1 gp140 trimer.